docs: add 23 new documentation pages for undocumented ENgrid features

[bryancasler]

Added documentation for scripts in the engrid repo that had no corresponding documentation in engrid-docs. Also updated the navigation in Layout.jsx to include all new pages in the appropriate sections.

New Documentation Files (23):

Form Features (7):

• validation.md — EN validators, postal code validation, min/max amounts, capitalize fields
• input-placeholders.md — Configurable placeholder text for form fields
• country-options.md — Auto country select, country redirect, country disable, expand region name
• url-parameters.md — URL to form, URL params to body attrs, set-attr, US-only form
• data-attributes.md — Data attributes, data hide, data replace with merge tags
• iframe.md — iFrame communication and postMessage handling
• autosubmit.md — Automatic form submission

All Pages (8):

• media-attribution.md — Photo/video credit overlays
• show-if-amount.md — Conditional display based on donation amount
• thank-you-page-conditional-content.md — TY page conditional content
• live-variables.md — Live amounts, frequencies, currencies, swap amounts, submit button variables
• skip-link.md — Accessible skip-to-main-content link
• mobile-cta.md — Sticky mobile call-to-action button
• progress-bar.md — Animated multi-page progress bar
• ticker.md — Animated scrolling text display

Donation Pages (3):

• digital-wallets.md — Google Pay, Apple Pay, PayPal, Venmo, DAF integration
• stripe-financial-connections.md — Stripe bank account verification for ACH
• premium-gift.md — Premium gift page enhancements

Advanced Features (4):

• email-validation.md — NeverBounce and FreshAddress email validation
• universal-opt-in.md — Consistent email opt-in across page types
• fast-form-fill.md — Hide pre-populated form sections
• event-tickets.md — Event ticket currency formatting

Core Concepts (1):

• debug-panel.md — Debug panel for development and testing

Navigation Updated:

• Added all 23 new pages to Layout.jsx navigation in appropriate sections

Scripts still needing investigation:
supporter-hub.ts, tidycontact.ts, vgs.ts, a11y.ts, add-name-to-message.ts, advocacy.ts, amount-label.ts, branding-html.ts, give-by-select.ts, src-defer.ts, cookie.ts, loader.ts, logger.ts, ecard.ts, show-hide-radio-checkboxes.ts, show-if-present.ts, set-recurr-freq.ts, custom-currency.ts, auto-year.ts, checkbox-label.ts, translate-fields.ts, expand-region-name.ts, frequency-upsell-modal.ts, other-amount.ts, page-background.ts, input-has-value-and-focus.ts